This is the heatsink(s) from the 5U 'BIG Amp Chassis' It has a mirror-imaged set of pre-drilled heatsinks and brackets to hole them together and make a mounting point for the rest of the enclosure. The 4U is similar, but the heatsink is a single piece.

Of note, I got the bigger (Dale/Vishay RN60) resistors to see how they would fit on the PCB. They are the size of the PRP resistors that are quite popular amongst the fancy parts crowd. They are great everywhere except the row flanking the small transistors right in the middle. They don't fit there side by side. You could mount them soldier style, or just mix in a few smaller resistors like I did. Or just get RN55's. They are the smaller size.

http://i198.photobucket.com/albums/a...ps616b983c.jpg
Here is a photo of the PSU board, I am going to use integrated bridge rectifier blocks, so you need to remove the part of the PCB that mounts the diodes. The new PCB, not quite yet available at the time of this writing, will have a similar feature with the diodes, as well as room for more/bigger capacitors.

http://i198.photobucket.com/albums/a...ps595e76c4.jpg
Here you can see the bridges with the wires attached from the transformer secondary. Remember that the green attached to a bridge must have continuity with the blue attached to the same bridge. (As it's the 2 ends of the same piece of wire)

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As long as were are tailing about the transformer, here is a photo of the terminal block shown wired for 120v. The Blur lead is the AC Live, and the clear the AC neutral. The reds and blacks are the transformer primaries.

The last bit of the PSU wiring is the chassis connection, the black comes from the PSU GND, and the green is the AC safety earth.

The AC to primary wiring confuses everybody, so;

Let's look at the PSU schematic just to make sure everything is OK... Remember that I am wiring it for 120v operation, so the transformer primaries are in parallel. People wiring for 240 with a transformer like this, please ignore.

If you look at the red and black wires in the photo you will see that the Mains AC must to pass through a thermistor to connect to each of the 2 primaries. And that is the point of them, to keep inrush under control during powerup.

http://i198.photobucket.com/albums/a...ps250cb1fc.jpg
A few notes on bias -
P1 controls the bias, measured across any of the 3W source resistors. Adjust for 0.13v when it's cold, and watch that it doesn't get higher than 0.2v once it's up to temperature in about an hour. Adjust for 0.2v when hot.

I'm driving the F4 with an O2 Headphone amp sourced from an iPod; Driving 85.5db speakers. Although it is a small room, it gets louder than I want to listen. It still can't drive it to clipping, but it does get really, really loud.

One thing worth mentioning, and it speaks very highly to the quality of the amp, is that it is completely non-fatiguing, and more interestingly, very easy to listen to turned up too loud… I don't realize how loud it actually is sometimes. Complete transparency is a word used a lot when describing this amp - but I have to agree. It's fantastic!

Please comment away if you desire.

Also please feel free to ask any F4 questions here, and if you would like to post photos of your F4 completions, old or new, please do!

http://i198.photobucket.com/albums/a...ps03c5379d.jpg
That completes it, the chassis is quite simple to assemble, and the pre-drilled back makes things really simple.
The amp goes together with very little effort -- it's a rewarding project.
You can also see in these photos that the 5U 'Big Amp Chassis' is, in fact, as advertised, being quite enormous.
I used the 5U because I had it, and it will be used for a number of these guides. The 4U 'Jack of all Chassis' would be plenty for this amp.

http://i198.photobucket.com/albums/a...psf5829d1c.jpg
For now I am driving the F4 with an O2 Headphone amp, as it is the only 'preamp' I have that can swing enough volts (7) to give real volume, although not to clipping. Still, has enough drive for my current setup, so I'm not wishing for something else. It also sounds fantastic. Seriously.
An ImPasse will be built to drive this amp, and I am also very interested in the Pumpkin / Shunty.

So how does it sound? Well, there is not a lot to say because it's so incredibly neutral, it sounds a lot like the O2, which is very neutral and musical itself.

I have been finding the amp extremely difficult to listen to critically and listen to the amp, as I find myself singing along to whatever I play… Every time. It's really cool.

How does it compare to the F5? That's very hard to describe - you the F5 is a fantastic sounding amp, no question. Better than any other amp I have ever built, tube or SS. It (the F5) will make you understand the whole 'Class-A' mystique, and you will be very happy that you built it, because it delivers on it's promises. It truly is a great amp and a great design.

The F5, when compared to the F4 is, in my opinion, is a bit boring. The F4 is that good. . But remember it comes at a price, specifically that you will likely need a dedicated preamp for it. (Or you could bi -amp it with a flea-powered tube amp, but that's a different story. See the manual for more info.)

Some interesting measurements concerning bias levels -

Here are some quick and dirty measurements I did looking at various levels of bias on the F4. ( +/- 22.5V rails )

http://i198.photobucket.com/albums/a...ps1060c074.jpg
FFT of the distortion residual. (please excuse the photo of the screen. The next time I do this I will get screenshots.) The 1st peak at 2.2k is the fundamental. (2.2k chosen as it aligns with the gridlines. ) Looking right, he 2nd peak is the 2nd harmonic, the 3rd peak the 3rd harmonic, and the 4th peak the 4th harmonic. Higher level harmonics are not visible, as they are very small and lost in the noise.

Now let's increase the bias to the recommended amount, 200mv measured across the source resistors.

So the logical thing is to just crank it all the way up, right? Well, yes and no. Here are the reasons (and people with more experience please correct me if I'm wrong.)

1) Heatsink - at some point you are going to get too hot. With the 5U that wasn't an issue. A good rule of thumb is the transistors 65C max and heatsinks 55C max.

2) Bias current vs. VA of transformer. You don't want your total bias current (in watts) to be more than 1/2 or maybe 2/3 or your transformer's VA rating.

3) Transistor dissipation. Make sure you look at the datasheet for the output transistors, they all have an absolute maximum and a de-rating as they get hotter. For long life don't set more than 1/2 the max dissipation.

4) Point of diminishing returns. In this example the measured distortion continued to decrease (down to .045 or so) as bias was increased to 370mV, but the gains were very small and it would added lots of heat. The sweet spot was closer to 300mV bias.

PKI

17th April 2013 10:46 PM

Beautiful! as always :-)

zany

18th April 2013 01:30 AM

awesome build guide... again! many thanks for all your efforts.

bnorrish

18th April 2013 01:39 AM

:p Awesome build. I should give this a shot - easy to build and swap out with my F5. Preamp outs on my Yamaha receiver would probably drive but the DIY Store could use a pre-amp board with some gain. ;)

buzzforb

18th April 2013 01:52 AM

There are. BA1/2 and BA3. Both are very capable and can easily work as a preamp.

6L6

18th April 2013 02:02 AM

I would agree with Buzz - I looked very closely at the BA3 front-end as a preamp before deciding on the ImPasse... which was decided on for a bunch of non-performance related reasons. (Although it's performance is no slouch at all...)

Specifically, I am interested in a project with an input transformer, I have a ton of very premium 6SN7 and 6DJ8, I haven't built a tube project in a while, etc...

bnorrish

18th April 2013 02:13 AM

Quote:

Originally Posted by buzzforb
(Post 3458687)

There are. BA1/2 and BA3. Both are very capable and can easily work as a preamp.

Interesting - board is $7. I have not looked the the BAs that closely. Anyone post one of those to make standalone preamp? Need to add some sort of attenuation for volume. I am sure there are a few other details.

6L6

18th April 2013 02:28 AM

Like anything, put your selector switch then your pot/stepped attenuator before the input of the gain stage. Easy!

PSU could be done a million different ways, choose your preferred flavor.

I would be happy to help, if you would like a nudge in the right direction. :)

thor61

18th April 2013 04:48 AM

1 Attachment(s)

Really great pics and write-up ! I have a heat-sink question as I have not drilled my sinks yet. Could someone tell me the distance from the bottom for the mosfet bolt holes ? See pick Attachment 343260